thx . hmm good point about the wall plug.....I don't think there are led's that can take that voltage. I'm guessing that there is a circuit of some kind that steps down the voltage. maybe an easy hack. they aren't that expensive so you might want to try it.

So, lets say you have a battery of 9vYou run 5v(depending on the threshold you choose for the amount of light) to the Positive(+) pole of the comparator. Then you would put the photocell in series between the battery and the Negative(-) pole of the comparator. During the day or lighted conditions, the photocell will have less resistance therefore letting more voltage through to the comparator. During lighted conditions, that of course depending on the threshold you choose, the photocell will let more than 5v through to the negative poll, therefore the output bit would be Zero(0). During darker conditions, the photocell will resist the voltage more and thus less than 5v will get to the comparators negative poll resulting in the output bit being One(1) which, if im not mistaken, should be 5volts output. I guess that would depend on the source voltage though. Then just run that 5v directly to the LED but through a resistor so that only about 1volt gets to the LED to extend its life time... or 2 volts if you want it to be much brighter but not last as long.

You could also use an undervoltage monitor. Its a 3 pin package where +v goes in 1 end and gnd at the other. the third pin depends on what type you want so voltage will flow out of the 3rd if the input voltage is above or below a certain threshhold depending on the type you get. Most companies with samples programmes offer them

p.s. There could also be a market for well designed battery powered night lights since it is now recommended for parents of young children not to use the plug in variety since it encourages children to the plug sockets